I just upgraded my Ubuntu 11.10 (x86) to Ubuntu 12.04 (x86). The upgrade completed without any errors.

Since then, I am unable to access any of the applications in the Dash. The Dash seems to be empty. When I search for some application like gedit, it says "Sorry, there is nothing that matches your search".

I had a fresh install of 12.04, not an upgrade. Just wanted to point that out since someone else had said that one of the previous answers was for upgrade path only. These packages were not installed. Once installed, all was well with the world.
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twindhamFeb 7 '13 at 19:23

These packages seem to be unavailable on 14.04.
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i08inJun 16 '14 at 12:47

Same here. After upgrading from 11.04 to 11.10 to 12.04 (beta2) the DASH menu doesn't list any applications. Previously typing "calc" for example would list the calculator and LibreOffice Calc. Now, nothing shows up, neither in the main DASH window, nor in the application lens.

However, in the main lens songs (FLAC) do show up, but for instance documents don't. Furthermore, I have regular crashes of unity-lens-video. Apport told me this problem had already been reported. Maybe this is related?

Looking a bit closer in ~/.xsession-errors I found that the zeitgeist daemon crashed:

I just encountered this problem earlier today on my laptop. It surprised me, given that I couldn't think of any reason it didn't show up until now.

After some struggling, I eventually resolved the issue by opening a terminal (Ctrl+Alt+T if you don't already have it pinned to the launcher) and using unity --replace &. (You could probably also do this with Unity's Alt+F2 prompt, but it too went missing for me at the time.)